Gauge using environment-insensitive radiation beam cross-section limiter

ABSTRACT

Gauge for measuring the characteristics of a specimen including a shielded source of radiation, a detector spaced from said source, a radiation beam cross-section limiter through which radiation from the source passes as it travels along a path for impingement upon the specimen which is located in a gap between the limiter and the source for selectively blocking and unblocking radiation from the source. The radiation beam crosssection limiter includes a bore sealed at both ends through which the radiation from the source passes in its path to the specimen and detector. By virtue of the sealed beam-limiting bore the mass of material contained within the bore, which usually is air, remains constant independent of changes in the environment, such as changes in environmental temperature, barometric pressure, humidity and the composition of the surrounding gas, thereby substantially reducing errors in the gauging process introduced by such environmental changes.

United tates Patent [1 3,668,399 Cahiii et al. June 6, 1972 [54] GAUGEUSING EONNIE Primary Examiner-Anthony L. Birch INSENSHTIVE RADIATIONBEAM CROSS-SECTION LIMITER Bonaventure B. Cahill, Ft. Mitchell, Ky.;Wilfred W. Lyon, Cincinnati, Ohio The Ohmart Corporation, Cincinnati,Ohio Filed: June 30, 1969 Appl.No.: 837,681

Inventors:

Assignee:

0.5. Ci. ..2so/a3.3 1), 250/105 1m. c1. ..c21r 5/02, n01 j 35/16 Fieldof Search ..250 83.3 D, 49.5 TE, 105

References Cited UNITED STATES PATENTS Boyd ..2so/s3.3 D.

Attorney-Wood, Herron & Evans ABSTRACT Gauge for measuring thecharacteristics of a specimen including a shielded source of radiation,a detector spaced from said source, a radiation beam cross-sectionlimiter through which radiation from the source passes as it travelsalong a path for impingement upon the specimen which is located in a gapbetween the limiter and the source for selectively blocking andunblocking radiation from the source. The radiation beam cross-sectionlimiter includes a bore sealed at both ends through which the radiationfrom the source passes in its path to the specimen and detector. Byvirtue of the sealed beamlimiting bore the mass of material containedwithin the bore, which usually is air, remains constant independent ofchanges in the environment, such as changes in environmentaltemperature, barometric pressure, humidity and the composition of thesurrounding gas, thereby substantially reducing errors in the gaugingprocess introduced by such environmental changes.

12 Claims, 1 Drawing Figure 24a 34 Sal/FEE PATENTEDJUH '6 1972 GAUGEUSING ENVIRONMENT-INSENSI'ITVE RADIATION BEAM CROSS-SECTION LIMITER Thisinvention relates to radiation gauges adapted to measure the propertiesof a specimen on which the radiation impinges, and more particularly toradiation'gauges which are rendered substantially independent offluctuations 'in the environment in which the gauge is used.

In many gauging applications wherein a beam of radiation from a sourceis impinged upon a specimen whose characteristics are to.be measured, ithas been found desirable to limit the cross-section of the beam ofradiation which irradiates the specimen. In such applications where beamcrosssection limitation is desirable, the source of radiation istypically spaced from the specimen a distance ranging from 1 to 4 inchesand the radiation emitted by the source passed through a suitable beamcross-section limiting device, such as an elongated hollow tube,interposed between the source and specimen. The radiation from thesource, in the course of traveling through the tube in its path from thesource to the specimen, has its cross-section limited by interactionwith the internal walls of the tube.

Typically, the beam cross-section limiting tube communicates with theenvironment of the gauge. At one end the tube freely communicates withan air gap in which the specimen is positioned, defined by the beamcross-section limiter and a suitably disposed detector which receivesradiation transmitted through the specimen. The other'end of the beamcross-section limiter is positioned adjacent the source to permitcapture, or entry, of radiation emitted by the proximately locatedsource for transmission to the specimen.

One very serious problem associated with gauge configurations andgeometries of the type described, wherein a hollow beam cross-sectionlimiter tube is interposed between the radiation source and the air gapin which the specimen is located, is that certain of the parameters ofthe material in the beam limiting tube, which typically is air, varymarkedly with respect to certain reference parameters for which thegauge is standardized or calibrated. These parameters of the material inthe tube subject to variation include temperature, barometric pressure,humidity, and composition. Of these parameters, one of the moreimportant and, accordingly one which is discussed below in detail forillustrative purposes, is' the variation in tube air temperature.

Variations in tube air temperature are primarily'caused by departures intemperature of the specimen from the reference temperature. For example,in one particular application wherein the gauge is used to measure thethickness of paper sheet stock typically having a temperature ofapproximately 165 R, if the gauge is standardized or calibrated at thebeginning of a paper stock run before the gauging components andenvironment, including the air in the beam cross-section limiting tube,have come to thermal equilibrium with the paper being measured, thetemperature of the air in the beam limiter tube can change by as much as90 F. the course of the measuring operation.

The problem with changes in temperature of the air in the beamcross-section limiting tube is that the mass per unit area of the columnof air in the cavity defined by the interior of the tube, and throughwhich the beam of radiation emitted by the source must pass in its pathof travel to the specimen, changes at the rate of approximately 0.3percent per C. For a 2 k inch air column, in which the mass per unitarea of air column is 7.67 mgJcm the change in mass per unit area forthe air column is 0.023 mg./cm per C. In a gauging application of thetype described for measuring the thickness of paper sheet stock where,for example, the full scale deflection is 0.9 mils or 3.2 mg./cm theerror introduced into the thickness measurement is approximately 0.72percent per C. If the temperature of the air column changes by 50 C.,from the temperature at which the gauge was standardized or calibrated,which is not unusual in paperthickness applications of the typedescribed, an error of approximately 36 percent is introduced into thethickness measurement.

(50 C.) during It has been an objective of this invention tosubstantially eliminate, in radiation gauges of the type utilizinghollow beam cross-section limiting tubes or the like which communicatewiththe environment, gauging errors introduced by changes in parametersof the air column in the beam limiting tube from those at which thegauge was standardized or calibrated. This objective has beenaccomplished in accordance with the principles of this invention byutilizing an extremely simple, but extraordinarily unobvious, approachwhich is predicated upon the concept of interposing means in the'beaminglimiting tube to render the mass of the material which is in the tubesubstantially insensitive to variation in the environment, such asvariations in environmental temperature, barometric pressure, humidityand gas composition. In accordance with a preferred arrangement, theinterposed means is a fluid-tight'chamber filled with air through whichthe radiation beam passes in the course of having its crosssectionlimited. Although certain of the parameters of the air in the chamber,for example, its temperature, vary as the ambient temperature changes,the mass of the air is invariant. Thus, gauging errors of the typenoted, wherein the mass of the air column in the beam limiter varieswith environmental changes, such as temperature changes, aresubstantially eliminated.

It has been a further objective of this invention to provide, in a gaugeof the type utilizing a sealed beam cross-section limiting chamberinterposed between the radiation source and detector and furtherutilizing a movable shutter to selectively block radiation, means forprotecting the gauging components from contamination by foreign mattersuch as dust and dirt in the environment. This objective has beenaccomplished by enclosing the source and operatively positioned shutterin a housing in which is provided therein an aperture which receives thesealed beam limiting chamber and thereby effectively seals the housing,and hence, the source and shutter, from the environment.

These and other advantages and objectives of the invention will becomemore readily apparent from a detailed description thereof taken inconjunction with the drawings in which the single FIGURE is a verticalcross-sectional view through the center of the detector, beam limiter,shutter and shielded source.

With reference to the FIGURE, a preferred embodiment of radiationgaugeincorporating the principles of this invention is seen to include asource of radiation 10 and a radiation detector 12. The detector 12 ispositioned to receive radiation from the source 10 which is transmittedthrough a specimen 14, whose characteristics are to be measured,interposed between the source and detector in an air gap 16. A radiationbeam cross-section limiter 18 is positioned between the air gap 16 andthe source of radiation 10 for restricting or limiting the cross-sectionof the radiation beam impinging upon the specimen 14. A shuttermechanism 20 having an aperture 22 selectively register-able with thesource 10 and beam cr0ss-section limiter 18 is provided to control theimpingement of radiation from the source 10 upon the specimen 14.Lateral and lower radiation shields 24 and 26 mounted on a frame 28partially surround the source 10 to prevent harmful radiation emitted bythe source 10 from reaching the gauge surroundings or environment. Ahousing 30 having an aperture 32 into which the beam cross-sectionlimiter l8 fits encloses the source 10, lateral and lower shields 24 and26, shutter mechanism 20, and frame 28.

The source 10 may take a variety of forms depending upon the thicknessand composition of the specimen 14 whose pro-" tured edge portions toaccommodate screw to the source to facilitate mounting.

The lateral shield 24 is preferably comprised of an outer lateral shieldmember 24a having a square horizontal crosssection, and winner lateralshield 24b having an annular horizontal cross-section. The inner shield24!: is snugly positioned in a bore 33 formed in the outer lateralshield 24a, and the source 10 is positioned within a bore 34 formed inthe inner annular shield 24b. Suitable screw fasteners 35 secure theannular shield 24b in the bore 33 of the outer shield 24a. When sosecured the lower surface of the annular shield 24b contacts a shoulder36 formed in thebore 33 to locate the upper surface of the inner shieldflush with the upper surface of the outer lateral shield 24a. Thecircular shoulder 36 also locates the source 10 by virtue of themounting plate 31 which is secured in contact with the lower surfacethereof by fasteners 38. The lower shield 26, like the lateral shield24, preferably has a square horizontal cross-section. Suitablemeans,(not shown) secure the lateralshield 24 to the lower shield 26which in turn is secured to the frame 28. The shields 24 and 26 arepreferably fabricated of lead.

The shutter mechanism includes a planar horizontal upper plate 40 inwhich is formed the aperture 22. A side plate 41 as well as a rear plate42 and a front plate (not shown) fasteners is fixed cross-section of theradiation beam is rendered fluid-tight. With the cavity 65 fluid-tightthe mass of the material within the cavity 65 remains constantindependent of fluctuations in the environment, such as fluctuations inenvironmental air pressure, humidity, barometric pressure or gascomposition.

, With the mass of the material in the cavity 65 independent of disposedparallel to the rear plate are integral with and extend verticallydownwardly from one side edge and the rear and front edges,respectively, of the plate 40. The plate 40 is bidirectionally movablein a horizontal plane in the direction of arrows 43 and 44 between aradiation blocking position (shownin solid lines) wherein the aperture22 fonned in the plate 40 is not registered with the bore 34 and aradiation unblocking position (shown in phantom lines) wherein theaperture- 22 is registered with the bore 34. To facilitate bidirectionalmovement of the plate 40 a solenoid 45 is provided having a horizontallyextending core 46 secured to the side plate 41 via a flange mount 39.

When the solenoid 45 is in the de-energized condition the core 46'andplate 40 are positively located in the position shown in solid lines bya compression spring 47 which is located in a cavity 53 formed in lowershield 26. The compression spring 47 is mounted on a horizontal shaft 48between a collar 50 fixed to the shaft 48 anda stationary shoulder 51surrounding a bore 52 which slidably receives one end of the shaft.Shaft 48 is secured to the free end of the core 46 via a pin 49. A guidebushing 55 is secured to the lower shield 26 and is provided with a bore57 which slidably receives the other end of the shaft 48.

In operation, when the solenoid is energized the core 46.

is retracted and shifts laterally in the direction of arrow 44compressing the spring 47 to move the aperture 22 into registry with thebore 34 and allow radiation from the source 10 to impinge upon thespecimen 14.

The thickness and composition of the plate 40 is selected to blockradiation from the source 10 when plate 40 is in the radiation blockingposition shown in solid lines.

The beam cross-section limiter 18 includes a bushing 60 having acircular cross-section bore 61 formed therein which defines acylindrical cavity or chamber 65 through which a beam of radiation fromthe source 10 is adapted to pass and the beam cross-section thereoflimited. The upper and lower ends of the bore 61 are sealed by radiationpenetrable windows 63 and 64. Preferably the windows 63 and 64 arefabricated of ,1 mil thick polyethylene terephthalate film which havetheir marginal edge portions adhered to the surfaces 58 and 59 of thebushing 60 with which they mate. A suitable adhesive such as thatmarketed by Armstrong Products Co., Inc., Warsaw, Indiana, designatedArmstrong C-9 Adhesive," has been found to satisfactorily bond the filmwindows 63 and 64 to bushing end surfaces 58 and 59, respectively. i v

With the upperand lower-ends of the bore 60 of beam cross-sectionlimiter 18 sealed by radiation penetrable windows 63 and 64,thecylindrical cavity 65 which limits the environmental changes, gaugingerrors aresubstantially reduced in gauges of the type disclosed whereinbeam crosssection limiters are utilized. In a preferred form of beamcrosssection limiter 18 the cavity 65 is filled with air. However, ifdesired, the cavity 65 may be evacuated, that is, placed under a vacuum,or alternatively, partially or completely filled with some othersuitable material. If desired, sealing of the cavity 65 can be dispensedwith altogether by filling the cavity with a low density solid or liquidmaterial whose mass does not change. The only restriction on thematerial located in cavity 65, whether or not it is sealed, is that itpermits a usable amount of radiation from the source 10 to impingeuponthe specimen 14 so as to permit a useful signal to be derived fromthe detector 12.

' The bushing has a radially extending circular flange 68. Flange 68seats on a circular lip 69formed in the housing 30 which defines thecircular aperture 32. Suitable fasteners '70 secure the flange 68 inplace on the lip 32 and effectively seal the housing aperture 32. Withthe housing aperture 32 sealed, dust and other foreign matter isprevented from reaching the various components of the gauge which areenclosed within the housing, such as the shutter mechanism 20, source 10and shields 24 and 26.

With the various components of the gauge of this invention in theposition shown in solid lines in the figure, the gauge'is in itsinoperative condition. In the inoperative condition the plate 40 ispositively driven by the compression spring 47 to the radiation-blockingposition wherein the aperture 22 formed in the plate 40 is notregistered with the bore 34 in which is located the source 10. With theplate 40 so positioned radiation from the source 10 is blocked by theplate and no radiation passes through the beam cross-section limiter 18to impinge upon the specimen 14.

- To render the gauge operative the solenoid 45 is energized. When thesolenoid is energized its core 46 moves to the right overcoming theforce of the spring 47.. .With the core 46 in its right-most positionthe shutter plate 40 is moved to the radiation unblocking position shownin phantom lines wherein the aperture 22 is registered with the bore 34surrounding the source 10. Radiation from the source 10 passesthroughthe aperture 22 into the cavity where the beam cross-section islimited. The cross-section limited radiation beam emanating from thewindow 63. of the limiter l8 impinges upon the specimen 14, in turnproviding a useful radiation input to the j detector 12.

Following calibration of the gauge in any well-known manner, should achange occur in the cavity 65, such as the temperature of the cavityvary, due, for example, to changes in the temperature of the specimen14, the mass of the material in the beam limiting cavity 65 does notchange and thereby introduce error into the gauging process. Bycontrast, if the mass of the material in the beam limiting cavity 65were not made independent of temperature by, for example, sealing thecavity 65, enormous errors could be introduced into the gaugingoperation by variations of the temperature of the material in the cavityfrom the temperature at which calibration was made. For example, it hasbeen found that errors of approximately 36 percent are 'mtroduced intothe gauging process by a a 50 C. temperature change when a gauge, whichhas a Kryptonsource and a 2 h inch long unsealed cylindricalbeamlimiting cavity, is used to measure the thickness of paper sheetstock having a nominal thickness of 0.9 mils. However, by sealing ormaking fluid-tight the beam-limiting cavity, and thereby rendering themass of the material enclosed therein constant and independent oftemperature variations, errors in the gauging process introduced by a 50C. change in temperature of the material in the beam-limiting cavity aresubstantially eliminated.

Departures in temperature of the air in the gap 16 due, for example, tovariations in temperature of specimen l4, operate to introduce slighterrors into the gauging process notwithstanding the use of thefluid-tight beam-limiting chamber 65. However, due to the relatively theair in the gap 16 relative to the mass per unit area of the specimen 14,any such error due to departures in temperature of the air in gap 16from the temperature at which calibration was made are kept tomanageable proportions.

An advantage of this invention attributable to the unique interrelationof the aperture housing 30, limiter l8, shutter 20 and shielded sourceis that the entire assembly is fully enclosed, thereby preventing theoperative components from being exposed to foreign matter in theenvironment such as dust or the like. Additionally, there are noexternally located moving parts to complicate the problem of sealing thehousing 3 The invention has been described with respect to avoidinggauging errors due to variations in the mass of the material in aradiation beam cross-section limiter, such as a collimator, due to thevariations in the environment, such as fluctuations in environmentaltemperature, humidity, barometric pressure, and gas composition.However, the principles of this invention have utility in otherapplications. For example, the principles of this invention wherein aconstant mass, that is, a mass independent of environmental changes, isinterposed in the radiation beam between the radiation source anddetector, can also be utilized in off-sheet air gap standardizationapplications. In gauging applications of the general type described thegauge is periodically standardized when off-sheet," that is, when thegauge has moved off the specimen and the specimen being measured is notin the air gap. To facilitate such offsheet standardization, meanshaving a mass which is independent of environmental changes may beinterposed in the air gap, that is, interposed in the space between thecollimator, if one is used, and the detector, when the gauge isoff-sheet and the specimen not in the air gap. The constant mass meansinterposed in the air gap may take many forms and may, for example, bein the form of a fluid-tight chamber which is evacuated or filled with agas such as air. Alternatively, the constant mass may constitute someother means whose mass does not change with variations in theenvironment such as a low density solid or liquid.

Having described the invention what is claimed is:

l. A radiation gauge for measuring the characteristic of a specimencomprising:

a source of radiation spaced from said specimen for directing a beam ofradiation having a predetermined cross-section through a specifiedlength path toward said specimen,

means interposed between said source and said specimen having across-section and length at least substantially coextensive with thecross-section and length of said beam, said means having a volume andmass which does not vary significantly with variations in gaugeenvironmerit,

a detector adapted to receive radiation from said beam after saidradiation has interacted with said specimen, and

a frame mounting said source and detector in a predetermined physicalrelationship relative to each other which is independent of saidvariations in gauge environment.

2. The gauge of claim 1 wherein said interposed means includes afluid-tight chamber.

3. The gauge of claim 2 wherein said chamber is filled with gas whoseentrapped mass does not significantly change with variations in theenvironment of said gas.

4. The gauge of claim 3 wherein said gas is air.

5. The gauge of claim 2 wherein said chamber is evacuated.

6. The gauge of claim 1 further including a collimator associated withsaid source for establishing said predetermined beam cross-section uponexit therefrom.

small mass per unit area of 5 7. A radiation gauge for measuring thecharacteristic of a specimen,

a source of radiation spaced from said specimen for directing a beam ofradiation through a specified length path toward said specimen,

a detector positioned to receive said beam of radiation aftertransmission through said specimen,

means interposed between said source and said specimen through whichsaid beam travels throughout substantially the entire length of saidpath, said means having a volume and mass which does not varysignificantly with variations in gauge environment, and

a frame mounting said source and detector in a predetermined physicalrelationship relative to each other which is independent of saidvariations in gauge environment.

8. A radiation gauge comprising:

a source of radiation;

a detector spaced from said source,

a collimator associated with said source for directing a beam ofradiation along a specified length path from said source to saiddetector, and

a specimen interposed between said collimator, and said detector throughwhich said beam is transmitted, said specimen having a mass per unitarea which is approximately the same order of magnitude as the mass perunit area of a volume of air having the dimensions of said beam,

means interposed between said source and said specimen through whichsaid beam travels throughout substantially the entire length of saidpath, said means having a volume and mass which does not varysignificantly with variations in gauge environment, and

a frame mounting said source and detector in a predetermined physicalrelationship relative to each other which is independent of saidvariations in gauge environment.

9. The gauge of claim 8 wherein said collimator includes a through boreand wherein said constant mass interposed means is positioned withinsaid bore.

10. The gauge of claim 9 wherein said bore is gas-tight and wherein saidconstant mass interposed means is gas enclosed within said gas-tightbore.

1 1. The gauge of claim 10 wherein said gas is air.

12. Apparatus for use in radiation gauging of a specimen comprising:

a source of radiation,

a radiation shield fixed relative to said source for enclosing saidsource, said shield having an aperture therein through which a beam ofradiation from said source passes,

a radiation beam cross-section limiter associated with said source, saidlimiter having a bore for defining the crosssection of said beam as saidbeam passes therethrough along a specified path from a radiation entryend mounted adjacent said shield aperture to a radiation exit endmounted adjacent to said specimen, said bore ends being sealed to renderthe mass enclosed by said sealed bore substantially independent ofvariations in gauge environment,

a detector adapted to receive radiation from said beam after saidradiation has interacted with said specimen,

a radiation impenetrable shutter mounted between said shield apertureand beam limiter for movement relative to said aperture betweenalternatively beam blocking and unblocking positions,

a housing enclosing said source, shield, and shutter, said housinghaving an opening therein which receives said limiter therebyeffectively sealing said opening, and

' a frame mounting said source and detector in a predetermined physicalrelationship relative to each other which is independent of saidvariations in gauge environment.

1. A radiation gauge for measuring the characteristic of a specimencomprising: a source of radiation spaced from said specimen fordirecting a beam of radiation having a predetermined cross-sectionthrough a specified length path toward said specimen, means interposedbetween said source and said specimen having a cross-section and lengthat least substantially coextensive with the cross-section and length ofsaid beam, said means having a volume and mass which does not varysignificantly with variations in gauge environment, a detector adaptedto receive radiation from said beam after said radiation has interactedwith said specimen, and a frame mounting said source and detector in apredetermined physical relationship relative to each other which isindependent of said variations in gauge environment.
 2. The gauge ofclaim 1 wherein said interposed means includes a fluid-tight chamber. 3.The gauge of claim 2 wherein said chamber is filled with gas whoseentrapped mass does not significantly change with variations in theenvironment of said gas.
 4. The gauge of claim 3 wherein said gas isair.
 5. The gauge of claim 2 wherein said chamber is evacuated.
 6. Thegauge of claim 1 further including a collimator associated with saidsource for establishing said predetermined beam cross-section upon exittherefrom.
 7. A radiation gauge for measuring the characteristic of aspecimen, a source of radiation spaced from said specimen for directinga beam of radiation through a specified length path toward saidspecimen, a detector positioned to receive said beam of radiation aftertransmission through said specimen, means interposed between said sourceand said specimen through which said beam travels throughoutsubstantially the entire length of said path, said means having a volumeand mass which does not vary significantly with variations in gaugeenvironment, and a frame mounting said source and detector in apredetermined physical relationship relative to each other which isindependent of said variations in gauge environment.
 8. A radiationgauge comprising: a source of radiation; a detector spaced from saidsource, a collimator associated with said source for directing a beam ofradiation along a specified length path from said source to saiddetector, and a specimen interposed between said collimator, and saiddetector through which said beam is transmitted, said specimen having amass per unit area which is approximately the same order of magnitude asthe mass per unit area of a volume of air having the dimensions of saidbeam, means interposed between said source and said specimen throughwhich said beam travels throuGhout substantially the entire length ofsaid path, said means having a volume and mass which does not varysignificantly with variations in gauge environment, and a frame mountingsaid source and detector in a predetermined physical relationshiprelative to each other which is independent of said variations in gaugeenvironment.
 9. The gauge of claim 8 wherein said collimator includes athrough bore and wherein said constant mass interposed means ispositioned within said bore.
 10. The gauge of claim 9 wherein said boreis gas-tight and wherein said constant mass interposed means is gasenclosed within said gas-tight bore.
 11. The gauge of claim 10 whereinsaid gas is air.
 12. Apparatus for use in radiation gauging of aspecimen comprising: a source of radiation, a radiation shield fixedrelative to said source for enclosing said source, said shield having anaperture therein through which a beam of radiation from said sourcepasses, a radiation beam cross-section limiter associated with saidsource, said limiter having a bore for defining the cross-section ofsaid beam as said beam passes therethrough along a specified path from aradiation entry end mounted adjacent said shield aperture to a radiationexit end mounted adjacent to said specimen, said bore ends being sealedto render the mass enclosed by said sealed bore substantiallyindependent of variations in gauge environment, a detector adapted toreceive radiation from said beam after said radiation has interactedwith said specimen, a radiation impenetrable shutter mounted betweensaid shield aperture and beam limiter for movement relative to saidaperture between alternatively beam blocking and unblocking positions, ahousing enclosing said source, shield, and shutter, said housing havingan opening therein which receives said limiter thereby effectivelysealing said opening, and a frame mounting said source and detector in apredetermined physical relationship relative to each other which isindependent of said variations in gauge environment.